Mater et Magistra §229

²²⁹ The transition from theory to practice is of its very nature difficult; and it is especially so when one tries to reduce to concrete terms a social doctrine such as that of the Church. There are several reasons why this is so; among them We can mention man's deep-rooted selfishness, the materialism in which modern society is steeped, and the difficulty of determining sometimes what precisely the demands of justice are in a given instance.